Re: [PATCH 4.9 72/76] arm64: futex: Fix FUTEX_WAKE_OP atomic ops with non-zero result value

[Nathan Chancellor]

On Wed, Apr 17, 2019 at 08:15:08AM +0200, Greg Kroah-Hartman wrote:
> On Tue, Apr 16, 2019 at 09:47:51AM -0700, Nathan Chancellor wrote:
> > On Tue, Apr 16, 2019 at 11:00:52AM +0200, Greg Kroah-Hartman wrote:
> > > On Mon, Apr 15, 2019 at 03:01:51PM -0700, Nathan Chancellor wrote:
> > > > On Mon, Apr 15, 2019 at 08:44:36PM +0200, Greg Kroah-Hartman wrote:
> > > > > From: Will Deacon <will.deacon@xxxxxxx>
> > > > > 
> > > > > commit 045afc24124d80c6998d9c770844c67912083506 upstream.
> > > > > 
> > > > > Rather embarrassingly, our futex() FUTEX_WAKE_OP implementation doesn't
> > > > > explicitly set the return value on the non-faulting path and instead
> > > > > leaves it holding the result of the underlying atomic operation. This
> > > > > means that any FUTEX_WAKE_OP atomic operation which computes a non-zero
> > > > > value will be reported as having failed. Regrettably, I wrote the buggy
> > > > > code back in 2011 and it was upstreamed as part of the initial arm64
> > > > > support in 2012.
> > > > > 
> > > > > The reasons we appear to get away with this are:
> > > > > 
> > > > >   1. FUTEX_WAKE_OP is rarely used and therefore doesn't appear to get
> > > > >      exercised by futex() test applications
> > > > > 
> > > > >   2. If the result of the atomic operation is zero, the system call
> > > > >      behaves correctly
> > > > > 
> > > > >   3. Prior to version 2.25, the only operation used by GLIBC set the
> > > > >      futex to zero, and therefore worked as expected. From 2.25 onwards,
> > > > >      FUTEX_WAKE_OP is not used by GLIBC at all.
> > > > > 
> > > > > Fix the implementation by ensuring that the return value is either 0
> > > > > to indicate that the atomic operation completed successfully, or -EFAULT
> > > > > if we encountered a fault when accessing the user mapping.
> > > > > 
> > > > > Cc: <stable@xxxxxxxxxx>
> > > > > Fixes: 6170a97460db ("arm64: Atomic operations")
> > > > > Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
> > > > > Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > > > > 
> > > > > ---
> > > > >  arch/arm64/include/asm/futex.h |   16 ++++++++--------
> > > > >  1 file changed, 8 insertions(+), 8 deletions(-)
> > > > > 
> > > > > --- a/arch/arm64/include/asm/futex.h
> > > > > +++ b/arch/arm64/include/asm/futex.h
> > > > > @@ -33,8 +33,8 @@
> > > > >  "	prfm	pstl1strm, %2\n"					\
> > > > >  "1:	ldxr	%w1, %2\n"						\
> > > > >  	insn "\n"							\
> > > > > -"2:	stlxr	%w3, %w0, %2\n"						\
> > > > > -"	cbnz	%w3, 1b\n"						\
> > > > > +"2:	stlxr	%w0, %w3, %2\n"						\
> > > > > +"	cbnz	%w0, 1b\n"						\
> > > > >  "	dmb	ish\n"							\
> > > > >  "3:\n"									\
> > > > >  "	.pushsection .fixup,\"ax\"\n"					\
> > > > > @@ -53,29 +53,29 @@
> > > > >  static inline int
> > > > >  arch_futex_atomic_op_inuser(int op, int oparg, int *oval, u32 __user *uaddr)
> > > > >  {
> > > > > -	int oldval = 0, ret, tmp;
> > > > > +	int oldval, ret, tmp;
> > > > >  
> > > > >  	pagefault_disable();
> > > > >  
> > > > >  	switch (op) {
> > > > >  	case FUTEX_OP_SET:
> > > > > -		__futex_atomic_op("mov	%w0, %w4",
> > > > > +		__futex_atomic_op("mov	%w3, %w4",
> > > > >  				  ret, oldval, uaddr, tmp, oparg);
> > > > >  		break;
> > > > >  	case FUTEX_OP_ADD:
> > > > > -		__futex_atomic_op("add	%w0, %w1, %w4",
> > > > > +		__futex_atomic_op("add	%w3, %w1, %w4",
> > > > >  				  ret, oldval, uaddr, tmp, oparg);
> > > > >  		break;
> > > > >  	case FUTEX_OP_OR:
> > > > > -		__futex_atomic_op("orr	%w0, %w1, %w4",
> > > > > +		__futex_atomic_op("orr	%w3, %w1, %w4",
> > > > >  				  ret, oldval, uaddr, tmp, oparg);
> > > > >  		break;
> > > > >  	case FUTEX_OP_ANDN:
> > > > > -		__futex_atomic_op("and	%w0, %w1, %w4",
> > > > > +		__futex_atomic_op("and	%w3, %w1, %w4",
> > > > >  				  ret, oldval, uaddr, tmp, ~oparg);
> > > > >  		break;
> > > > >  	case FUTEX_OP_XOR:
> > > > > -		__futex_atomic_op("eor	%w0, %w1, %w4",
> > > > > +		__futex_atomic_op("eor	%w3, %w1, %w4",
> > > > >  				  ret, oldval, uaddr, tmp, oparg);
> > > > >  		break;
> > > > >  	default:
> > > > > 
> > > > >
> > > > 
> > > > This causes a (false) build warning with AOSP's GCC 4.9.4 (which is
> > > > used to build nearly all arm64 Android kernels before 4.14):
> > > > 
> > > >   CC      kernel/futex.o
> > > > ../kernel/futex.c: In function 'do_futex':
> > > > ../kernel/futex.c:1492:17: warning: 'oldval' may be used uninitialized in this function [-Wmaybe-uninitialized]
> > > >    return oldval == cmparg;
> > > >                  ^
> > > > In file included from ../kernel/futex.c:69:0:
> > > > ../arch/arm64/include/asm/futex.h:56:6: note: 'oldval' was declared here
> > > >   int oldval, ret, tmp;
> > > >       ^
> > > > 
> > > > The only reason I bring this up is Qualcomm based kernels have a Python
> > > > script that emulates -Werror, meaning this will be fatal for a large
> > > > number of kernels, when this eventually gets merged into them.
> > > 
> > > Argh, really?  That's a buggy compiler that you have there, as oldval
> > > will be set correctly if all is good, and if not, ret will be and the
> > > code will error out.
> > > 
> > 
> > Correct.
> > 
> > > Working around broken compilers is not something I really like doing :(
> > > 
> > 
> > Indeed, I wouldn't have brought it up if it wasn't the compiler for all
> > Android 4.9 kernels aside from the Pixel 3 (XL).
> > 
> > > That being said, does this also show up in the 4.19.y and 5.0.y tree
> > > right now?  If not, why not?
> > > 
> > 
> > It does.
> > 
> > $ make ARCH=arm64 CROSS_COMPILE=<path>/bin/aarch64-linux-gnu- defconfig kernel/futex.o
> 
> Great, so it seems this needs to be fixed in Linus's tree first, before
> I can backport it everywhere.
> 

Well, is it worth working around this in Linus's tree? I know you hate
taking patches just for stable but this compiler won't be used on 4.14+
according to [1] and support for it is planned to be discontinued in
less than a year [2]. This warning doesn't happen with Clang or newer
versions of GCC (I tested 6.3 in a Debian Docker image, which seems to
be the oldest I can find). I suppose there could be other buggy/ancient
compilers to work around...

> Want me to send a patch for this or can you?
> 

I am happy to send a patch regardless of where it goes, just want to be
sure we are all on the same page.

[1]: https://android.googlesource.com/platform/test/vts-testcase/kernel/+/e1622ae19e0419ceac363d31f7fd53b514b8c218
[2]: https://android.googlesource.com/platform/prebuilts/clang/host/linux-x86/+/a28e116f380a7e7c19e4639b6a35fecf5dddd4e8

Thanks,
Nathan

> thanks,
> 
> greg k-h